Do you absorb calories if lactose is intolerant?

December 31, 2025 •

4 min read

According to the National Institutes of Health, about 65% of the global adult population has some form of reduced lactase activity, leading to lactose malabsorption. This common digestive issue raises a critical question for many: does my body actually absorb the calories from the lactose it can't digest?

Quick Summary

Lactose intolerant individuals do not fully absorb calories from lactose due to an enzyme deficiency. Instead, undigested lactose ferments in the colon, affecting overall calorie uptake and potentially causing discomfort.

Key Points

Incomplete Absorption: A lactose intolerant person does not absorb all the calories from the lactose sugar in dairy due to a lack of the lactase enzyme.
Bacterial Fermentation: Undigested lactose passes to the large intestine where it is fermented by bacteria, producing gas and short-chain fatty acids.
Partial Calorie Recovery: The body can absorb some of the calories from the short-chain fatty acids produced during fermentation, so not all calories are lost.
Other Nutrients Absorbed: Important nutrients like protein and fat from milk are still absorbed, though severe diarrhea can affect overall nutrient absorption.
Tolerance Varies: The amount of lactose that can be tolerated and the degree of calorie absorption varies significantly from person to person.

Understanding Lactose Intolerance and Digestion

Lactose intolerance is the inability to fully digest lactose, a sugar found in milk and dairy products. It stems from a deficiency in the enzyme lactase, which is responsible for breaking down lactose into simpler, absorbable sugars: glucose and galactose. Without sufficient lactase, undigested lactose travels through the digestive tract, where it causes a cascade of digestive symptoms.

The Path of Undigested Lactose

In a person with lactose intolerance, here is what happens to the unabsorbed lactose:

Travels to the Colon: Instead of being broken down and absorbed in the small intestine, the lactose moves on to the large intestine.
Bacterial Fermentation: The undigested lactose serves as food for the abundant bacteria in the colon. This process is known as fermentation.
Gas and Acid Production: The fermenting bacteria produce gases such as hydrogen, carbon dioxide, and methane. These gases lead to the common symptoms of bloating, flatulence, and abdominal pain. The bacteria also produce short-chain fatty acids (SCFAs), which are a small source of calories that the body can absorb.
Fluid Imbalance: Undigested lactose increases the osmotic pressure in the colon, drawing water into the bowel. This excess fluid contributes to diarrhea, a hallmark symptom of lactose intolerance.

The Truth About Calorie Absorption

The short answer to the question "do you absorb calories if lactose is intolerant?" is that you absorb fewer, but not zero. The calories associated with the lactose sugar are not completely lost. While the body cannot break down the lactose directly, the fermentation process in the colon does salvage some of the energy. The calories absorbed come primarily from the short-chain fatty acids produced by the gut bacteria during fermentation. The exact amount is highly individual and depends on several factors, including the severity of the intolerance and the efficiency of the individual's gut bacteria.

Comparison: Calorie Absorption with and without Lactase


Feature	Lactase Persistent (Tolerant)	Lactase Non-Persistent (Intolerant)
Lactase Enzyme	Sufficiently produced in the small intestine.	Produced in insufficient amounts.
Lactose Digestion	Lactose is broken down into glucose and galactose in the small intestine.	Lactose is not properly broken down and travels undigested to the large intestine.
Calorie Absorption	Nearly all calories from lactose are absorbed efficiently as glucose and galactose.	A significant portion of lactose calories is not absorbed. Some calories are salvaged from fermentation byproducts like short-chain fatty acids.
Symptom Profile	No digestive discomfort from consuming lactose.	Can experience bloating, gas, cramping, and diarrhea due to bacterial fermentation.

Does Lactose Intolerance Affect Other Nutrients?

It's important to distinguish between lactose and other nutrients in dairy products. Lactose intolerance specifically affects the digestion of milk sugar. The protein, fat, and other vitamins and minerals in milk are largely unaffected and can still be absorbed. However, severe digestive distress like diarrhea can cause a faster transit time for food through the gut, which may slightly reduce the overall absorption of other nutrients. This is not a consistent or significant issue for most people with lactose intolerance but can occur in severe cases.

Strategies for Managing Lactose Intake

For those who wish to include dairy in their diet, several strategies can help mitigate the effects of lactose intolerance and ensure proper nutrient absorption:

Lactase Supplements: Taking a lactase enzyme supplement, such as a chewable tablet, before consuming dairy can aid in the digestion of lactose.
Lactose-Free Products: Many brands now offer lactose-free milk, cheese, and yogurt, which are pre-treated with lactase and are nutritionally identical to their regular counterparts.
Low-Lactose Dairy: Hard, aged cheeses (like cheddar or parmesan) and yogurt often contain much lower levels of lactose due to the fermentation and aging process, making them more tolerable for many individuals.
Gradual Exposure: Some people can build a tolerance to small amounts of lactose over time. Starting with small servings and gradually increasing can help determine an individual's personal threshold.
Pair with Other Foods: Consuming dairy alongside a meal containing fats can slow down digestion and potentially lessen symptoms, giving the body more time to process the lactose.

For more information on managing symptoms and dietary changes, consult trusted resources like the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK): Eating, Diet, & Nutrition for Lactose Intolerance.

Conclusion

While a person with lactose intolerance doesn't absorb all the calories from the lactose in dairy, the total calorie loss is often not as dramatic as one might think. The undigested sugar is partially fermented by gut bacteria, which produces short-chain fatty acids that the body can use for energy. The primary impact is not a significant calorie reduction but rather the uncomfortable digestive symptoms that result from the fermentation process. Other valuable nutrients in dairy, like protein and fat, are still largely absorbed. By understanding the digestive process and using strategies like lactase supplements or lactose-free products, individuals with this condition can manage their symptoms while maintaining a balanced diet.

Frequently Asked Questions

The calories from undigested lactose are not absorbed in the small intestine. Instead, they travel to the large intestine, where gut bacteria ferment the lactose, producing gases and short-chain fatty acids. The body absorbs some calories from these fatty acids, but not the full amount.

Lactose intolerance is not a reliable method for weight loss. While you might absorb fewer calories from lactose, the amount is often not significant enough to cause substantial weight changes, and severe digestive symptoms can cause other health issues.

Yes, lactose intolerance primarily affects the digestion of milk sugar. The body still absorbs the protein and fat from dairy products. However, if you experience severe diarrhea, it could speed up intestinal transit and reduce the overall absorption of nutrients to a minor degree.

No, lactose intolerance and milk allergy are two different conditions. A milk allergy involves an immune system response to milk proteins, which can cause severe reactions. Lactose intolerance is a digestive issue caused by a lack of the lactase enzyme, and it does not involve the immune system.

Yes, lactose-free milk contains the same number of calories as regular milk. It is prepared by adding the lactase enzyme, which breaks down the lactose sugar. This makes the milk digestible for intolerant individuals but does not change its calorie content.

Taking lactase supplements helps your body digest the lactose properly, allowing you to absorb the calories from the sugar in your small intestine, just like a person without intolerance would. This prevents the symptoms and the partial calorie loss associated with fermentation.

Short-chain fatty acids (SCFAs), like acetate and butyrate, are produced when gut bacteria ferment dietary fibers and undigested sugars. The body's cells can absorb these SCFAs and use them for energy, making them a source of calories.